Mashable

A steam train carrying lumber stops on top of a trestle bridge on a tributary branch of the Northern Pacific Railroad.

Image: Corbis

As railroads spread westward across the United States in the 19th century, engineers had to contend with uneven, loose and rugged terrain. Because an undermined rail could lead to a literal trainwreck, tracks had to be level and secure.

When setting tracks across wide valleys, wooden trestles were often built to keep the track on a level course high above the ground. Most trestles were meant to be temporary, allowing trains to cart in materials needed to create a solid fill beneath the tracks.

Trestles were also used as permanent installations in areas where water flow or flooding might make solid fills unsafe.

Temporary trestles mostly fell out of favor in the late 20th century, when newer technologies made it cheaper to construct a fill without a trestle.

Wooden, steel and concrete trestles still remain common on railroads across the country.

c. 1863

A trestle bridge on the Orange & Alexandria Railroad in Virginia.

Image: Buyenlarge/Getty Images

1906

People sit on the front of a logging train as it crosses a trestle bridge in the Cascade Mountains in Oregon.

Image: Archive Photos/Getty Images

c. 1870

A locomotive engine pushes two open cars across a newly assembled railroad trestle in order to test it.

Image: Hulton Archive/Getty Images

c. 1900

A train passes over the Valley Trestle Bridge Chicago & North Western Railway viaduct over the Des Moines River, near Boone, Iowa.

Image:

1885

A train passes over the Dale Creek Bridge on the Union Pacific Railway in Sherman, Wyoming.